This invention generally relates to measuring current density in electrochemical processes, and more particularly relates to methods and apparatus for directly measuring current density in cathodic protection systems for the protection of large metal structures in contact with an electrolytic medium against corrosion attack. More particularly, this invention relates to methods and apparatus for directly measuring current density in any portion of a metal structure, even those having a complex geometric design and configuration, in contact with an electrolytic medium and protected by a cathodic protection system.
Large metal structures located in land or sea regions, such as pipelines, wells, structural supports, offshore drilling platforms, ship hulls, metal supports and framing under or imbedded in concrete, and the like, when exposed to a corrosive electrolytic environment tend to be corrosively destroyed over a period of time. To minimize this corrosive attack, many means have been utilized to safeguard the metal surfaces in contact with the corrosive electrolytic environment.
One such approach is to cover the metal structure with protective materials, such as inert wrappings of fiber or cloth, and exterior impervious coatings of bituminous materials, cement, epoxy resins and the like. Unfortunately, it is difficult to spread the protective coatings evenly during their application, or voids are created during the installation of the structure within the earth or sea region or thereafter by mechanical damage or injury to the structure and protective coating. The exposed surfaces of the structure are then exposed to the corrossive electrolytic environment.
Many schemes have been devised for protecting these structures. For example, cathodic protection systems have been developed for mitigating the corrosion of submarine or subterranean metal structures by connecting sacrificial anodes of a metal higher in the electromotive series than the metal of the structure, such as magnesium or zinc in the case of ferrous structures, to the structure and disposing them within the electrolytic environment or medium. Also, direct current electricity can be supplied to the structure to provide all or part of the current required for cathodic protection. Less expensive anodic metals, such as graphite, can also be utilized as anodes for an auxiliary current source as used to drive current from the anode to the structure being protected. Cathodic protection of the structure is achieved when cathodic areas of the structure receive all electrons utilized in the cathodic process from the auxiliary anode, and not from the local anodes of the structure itself.
In either of the above two events, the flow of cathodic current from the anode to the metallic structure is assumed to be of proper magnitude when the structure (steel) is about 0.80 to 1.00 volts negative relative to the earth region or medium immediately surrounding the structure as measured with respect to a silver-silver chloride reference electrode. Under these conditions, the structure may be directly exposed to the earth formation or sea water and all of its portions are protected against corrosion attack. The shift in cathode potential is proportional to the current density (mA/ft.sup.2) applied to the cathode. The desired current density (mA/ft.sup.2) for protecting offshore structures generally falls in the range of 1 mA/ft.sup.2 to 50 mA/ft.sup.2. However, a typical range for most structures is 5-10 mA/ft.sup.2. The current density required to move the potential into the protective non-corroding range varies widely with the grade of steel of the cathode, the electrolytic composition, temperature, pressure, flow rate of the electrolytic medium in sub-sea situations, etc. It is therefore desirable to be able to measure the current density accurately even if all of the factors are not known.
Many methods and arrangements have been proposed for evaluating the cathodic protection of a metal structure. Generally, these techniques only measure the potential of the structure in the earth region relative to a closely spaced adjacent reference electrode, such as the copper sulfate half-cell, or the silver-silver chloride reference electrode. Other techniques have been developed that monitor or measure the current passing into the anodes from the anode power supplies, and, assuming that all of the current flow is impressed upon the metal structure, an approximation of the current density may be calculated based on an approximation of the total surface area of the structure to be protected. However, this technique is only an approximation of the current density and it is known that the current density is not uniform over the entire structure surface.
Other techniques utilize a sample cathode spaced from and insulated from the metal structure and through which a "measurement" of the current impressed upon the sample cathode is made by means of determining the IR drop in a milliohm impedance shunt circuit. However, the accuracy of such a measurement is not reliable, because very low currents are hard to measure with any reliability when using such a low resistance shunt, and because low currents or current densities require a large cathode area, often 10-12 ft.sup.2 which has to remain perfectly insulated from the cathode. In a sea water environment, it is difficult to maintain the insulation intact when it is immersed in the seawater for a long time period.
In addition, no matter how accurate the actual readings of the current through the sample cathode are, since the current density determination is made by approximating the surface area of the metal structure, large errors can often result because of the difficulty in calculating the correct surface area of the many complex geometric configurations of certain structures, such as offshore oil well platforms and the like. In addition, it is well known that, because of the complex geometric configurations of certain structures, the current density is not uniform over the entire structure.
The monitoring and evaluation of cathodic protection systems of metal structures has been limited in the past by the inability to directly measure current density at any selected location on the surface of the structure. All attempts to measure current density have had to be confined to large areas because of the reasons previously outlined. Therefore, it has not been possible to monitor current density at what is essentially a "spot" location, which could be located in a confined area or shielded area of complex structure.
The present invention provides a method for directly measuring current density at any location on the surface of a metal structure protected by a cathodic protection system. The measuring apparatus is compact and may be installed in virtually any location on the surface of a metal structure, even including recesses and joints and shielded areas of structures having complex geometric configurations.